In aerobic biological processes the oxygen supply by aeration is of primary importance, but the low solubility of atmospheric oxygen in water, about 9 ppm at 20.degree. C., limits its transfer. Unlike the higher plants, most microorganisms lack the structures necessary for direct absorption of oxygen from air, and thus the supply of oxygen through the cell wall of the microorganism to the enzymatic system responsible for respiration must be accomplished through an external aqueous medium. Thus, this limited reservoir of dissolved oxygen is quickly depleted by actively growing aerobic microorganisms unless it is rapidly replenished by some source, typically by molecular diffusion from air. Further, the solubility of oxygen in typical industrial nutrient medium is even lower, sometimes half that in pure water, and can act as only a very small reservoir of available oxygen. Hence, oxygen supply and the efficiency of its method of distribution are typically limiting factors for the growth of aerobic microorganisms in liquid substrates. The efficient industrial cultivation of aerobic microorganisms is dependent on solving the problem of supplying oxygen at a rate sufficient to meet the inherent demand of the particular species of microorganism being cultivated on large scale.
Conventional equipment in which microorganisms are propagated generally consists of a vessel equipped with mechanical agitation and a means for introducing gases such as oxygen, air or carbon dioxide. Molecular transport between bubbles of the gas phase and the fermentation liquor is enhanced by mechanical agitation. The work expended to enhance molecular transport in this manner ranges up to 1 horsepower per 100 gallons of fermentation liquor. Additional energy must be expended in supplying the necessary gases which are conventionally introduced at a rate up to 1 volume of gas per volume of fermentation liquor per minute through a sparging system located below the mechanical agitator. Conventional equipment for promoting high rates of oxygen transfer to the liquid phase in chemical and biological reactors promote good gas distribution throughout the liquid phase by stirring. Such stirrers use various impeller designs alone or in combination with a draft tube, suction tube, baffles and other similar flow modification devices. In special circumstances efficient gas distribution is achieved utilizing air entrainment, such as by pumping liquid or gas-liquid mixtures through a jet or aspirator. Modified air lift systems have also been employed to enhance oxygen transfer in biological and chemical processes. The high cost of mechanical energy, combined with the high power input requirement per unit of oxygen transferred, and the high incidence of microbial contamination in gas sparged systems have a substantially adverse influence upon the economics of the conventional fermentation processes.
Further, the passage of large quantities of air through the fermentation liquors, accompanied by vigorous agitation, often produces large quantities of foam in the reaction vessel which severely limits the working volume of the vessel. The fermentation process can be rendered inoperable and microbially contaminated when the air flow exit lines become filled with foam.
Many chemical and mechanical devices have been proposed and developed to solve the foaming problem in industrial biosynthesis. Most existing methods are based on chemical or mechanical defoaming of an already developed foam. Chemical treatment currently used for defoaming typically involves silicones and other water-immiscible additives which substantially decrease the rate of oxygen transfer, thus interfering with the processes of aerobic biosynthesis. Mechanical defoamers which are sometimes used in fermentation processes require additional power supply and special fermentor design. In addition, their performance is not uniformly reliable and feasible, especially in large fermentation volumes.
The novel invention conceived in this disclosure circumvents both of the foregoing described deficiencies of conventional systems related to mass transfer and defoaming in biotechnology. This invention relates to a unique apparatus which facilitates the molecular transport between liquids and gases such as occurs, for example, in chemical reactors, bioreactors (fermentors), photobioreactors, natural or artificial ponds and facilities for cultivation of fish, of micro- and macro-flora and for waste treatment.